Disclosed herein are optically transparent, plastic, protective article assemblies, and specifically, optically transparent, weatherable, plastic that can protect optoelectronic devices, such as photovoltaic modules.
Photovoltaic (PV) modules have traditionally employed glass as a rigid superstrate, elastomeric encapsulation layers, and a polymeric backing sheet, making them relatively heavy and inflexible. More recently, flexible, lightweight modules have been made using thin film photovoltaic cells, such as amorphous silicon, and a transparent polymer film to allow light to enter and to protect the cell from the elements.
The transparent polymer film must meet a number of stringent requirements. It must have good transmission of light between the wavelengths of about 400 nm to about 1,200 nm and must protect the module components from physical damage and moisture. In particular, entry of liquid water into the module will inevitably cause corrosion and loss of performance. Cuts into the module can expose electrical components. Resistance toward cut damage is required by Underwriters Laboratories specification UL 1703 for Flat-Plate Photovoltaic Modules and Panels, for example. It is therefore essential that the polymer film covering the module be sufficiently tough toward physical damage. In addition, the film must retain its properties for long periods in use. Typically, lifetimes of twenty years or more are expected for photovoltaic modules. Few kinds of polymers have both adequate physical properties and sufficient resistance toward outdoor weathering under the high temperatures and full sun conditions of a photovoltaic module.
The current art employs a fluorocarbon polymer such as poly(ethylene-co-tetrafluoroethylene) (ETFE) available commercially as Tefzel* film from DuPont. Fluorocarbon polymers are extremely resistant toward weathering, and can easily endure twenty or more years in this application. However, fluorocarbon polymers are both relatively soft and relatively expensive. As a result, it is difficult to construct a module that has both low cost and has sufficient film thickness to avoid cut or puncture damage. One solution has been the introduction of glass fibers into the encapsulant layers to improve the cut resistance of a module based on an ETFE. While the glass fibers can protect against cuts deep enough to expose the electrical components and thereby pass the UL1703 standard, it is not clear that the integrity of the envelope toward water penetration is retained. Since these modules can be deployed directly on flat or angled roofs, damage is likely over the lifetime of the module.
Some photovoltaic cells require protection not just from liquid water but also from water vapor. Efficient cells can be made using cadmium telluride (CdTe) or from copper indium gallium selenide (CIGS), for example. However, these cells rapidly lose efficiency when exposed to water vapor. Since optoelectronic devices for outdoor applications, such as solar cells, currently carry a 20-25 year guarantee for outdoor use, these devices are protected from moisture, oxygen, and hail, with glass, which protects while at the same time still allowing solar spectrum to enter the device. Glass, however, is more rigid, brittle, and heavy.
Thus, there is a need for light weight (compared to glass) optoelectronic devices that are highly weatherable, tough, puncture resistant, and/or water vapor resistant.